As is well known in the art, such preforms or parisons can be produced with the aid of a vitreous supporting tube--generally of quartz--which is internally coated with vitrifiable reactants formed by the technique known as chemical-vapor deposition (CVD). The reactants usually include a silicon halide, a germanium halide, an oxidant and one or more doping agents whose relative proportions can be varied during deposition in order to establish a desired refractive-index profile. These constituents interact at an elevated temperature whereupon the resulting structure is thermally collapsed to form a solid rod; the latter is subsequently vitrified unless such vitrification has already occurred at an earlier stage.
When the preform thus obtained is drawn into a fiber, the tube material constitutes the cladding while the deposited substance becomes the core. The internal deposit may consist of 50 to 100 layers whose formation may require about 4 to 8 hours; the deposition of each layer is accompanied by a relative axial displacement of an external heater which locally increases the temperature of the tube to insure a fusion of the core material over its entire length. Thus, the tube undergoes numerous thermal cycles subjecting it to considerable stress. The thermal shock so exerted upon the tube locally reduces its viscosity and, on account of the existing surface tension, tends to cause its partial collapse in a zone of intensive heating. The resulting reduction of the inner diameter of the tube entails an uncontrollable throttling of the gas flow traversing same and, therefore, a change in the concentration of the dopants which can manifest itself in a deviation of the refractive index of the fiber from the programmed value. Once this constriction surpasses a certain limit, the process degenerates and yields an optically unsatisfactory preform.
The problem referred to is particlarly serious when the ingredients of the reactive mixture are such as to require high operating temperatures, e.g. above 1600.degree. C. Such high temperatures will be needed especially in the manufature of fibers that are to be used for signal transmission with wavelengths near the upper limit of the minimum-attenuation window lying in a range of 0.7 to 1.6.mu., e.g. with wavelengths above 1.3.mu.. Typical preforms for such fibers include those of graded refractive-index profile having a cladding of undoped silica and a core with a binary SiO.sub.2 /GeO.sub.2 matrix, operating with the lowest possible attenuation of 0.17 dB/Km for a wavelength of 1590 nm, as well as preforms of the monomode type with similar composition or with an SiO.sub.2 --F bond for the cladding and pure silica for the core.